Getting public ip address for a lan

Question

hello is their a way to some how retrieve the public ip address of a lan? When i use the standard java get host method from a pc that's in a lan, all i get is the private lan ip address such has 192.x.x.x. this is useless to me if i want to make a connection to this computer from outside the lan.
i want to be able to get the public ip so that i can transform my instant messenger program (LAN based) into the internet so that it clients can talk to eachother from all around the world like msn messanger.
would they be a problem if say 5 users from the same LAN where logged into the program and one person from outside the lan wanted to talk to one of the people from the lan? seeing as all 5 lan users will have the exact same public ip address, is their a way to talk to teh correct user?
The current state is that as soon as a user longs into the system, their ip address from the pc that they are using, is stored temporaily into a server so that if User A whats to talk to User B, User query the server to first find out if the user is online and than get their IP address and join User B,s listening socket using the ip address retieved from the server..

Answer 1

import java.net.URL;
import java.net.HttpURLConnection;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.InputStream;
String publicIP = null;
            try {
                URL tempURL = new URL("http://www.whatismyip.org/");
                HttpURLConnection tempConn = (HttpURLConnection)tempURL.openConnection();
                InputStream tempInStream = tempConn.getInputStream();
                InputStreamReader tempIsr = new InputStreamReader(tempInStream);
                BufferedReader tempBr = new BufferedReader(tempIsr);
                publicIP = tempBr.readLine();
                tempBr.close();
                tempInStream.close();
            } catch (Exception ex) {
                publicIP = "<Could-Not-Resolve-Public-IP-Address>";
            }